STM32 I2C Slave dirty trick

Context

The STM32L451 HAL doesn't directly support variable length message RX/TX.

Default HAL behavior

The defaut behavior is described in the GraphViz render.

Modification

void HAL_I2C_AddrCallback(I2C_HandleTypeDef *hi2c, uint8_t TransferDirection, uint16_t AddrMatchCode) {
  /* Slave matched his address */
  if (TransferDirection != 0) {
	  /* I2C Transmit buffer */
	  CSKB_I2C_Dir = I2C_TX; // Save in a global the current direction
	  uint8_t framesize = (CSKB_I2C_NXT_FrameSize == 0) ? I2C_BUFFER_SIZE : CSKB_I2C_NXT_FrameSize;
	  if (HAL_I2C_Slave_Seq_Transmit_IT(hi2c, (uint8_t *)CSKB_I2C_TX_BUFFER, framesize, I2C_FIRST_AND_LAST_FRAME) != HAL_OK) {
		/* Transfer error in transmission process */
		Error_Handler();
	  }
  } else {
	  CSKB_I2C_Dir = I2C_RX; // Save in a global the current direction
      /* I2C Receive on the rx buffer BUT we don't know the size beforehand */
	  if (HAL_I2C_Slave_Seq_Receive_IT(hi2c, (uint8_t *)CSKB_I2C_RX_BUFFER, I2C_BUFFER_SIZE, I2C_FIRST_AND_LAST_FRAME) != HAL_OK) {
		/* Transfer error in reception process */
		Error_Handler();
	  }
  }
}

Once we have those information we can try to use the final callback to handle everything we need.

void HAL_I2C_ListenCpltCallback(I2C_HandleTypeDef *hi2c)
{
	/* Handling here incomplete RX/TX with dirty hack */
	if (CSKB_I2C_Dir == I2C_TX) {
		CSKB_I2C_TX_Complete = 1;
		CSKB_I2C_TX_Size = (CSKB_I2C_NXT_FrameSize == 0) ? I2C_BUFFER_SIZE - hi2c->XferSize : CSKB_I2C_NXT_FrameSize - hi2c->XferSize;
	} else if (CSKB_I2C_Dir == I2C_RX) {
		CSKB_I2C_RX_Complete = 1;
		CSKB_I2C_RX_Size = I2C_BUFFER_SIZE - hi2c->XferSize;
	}
	CSKB_I2C_Dir = I2C_NONE;
}

For some reason it appears that XferSize member of the I2C handle has the remaining count of byte (starting from the buffer size declared while calling HAL_I2C_Slave_Seq_Receive_IT or HAL_I2C_Slave_Seq_Transmit_IT);